1. Technical Field
The present invention relates generally to Advanced Process Control (APC) applications, and more specifically relates to a system and method for optimizing metrology sampling in an APC application.
2. Related Art
As the ground rule requirements of semiconductor products tighten, the use of Advanced Process Control (APC) applications has increased in an effort to meet the improved process control needed to achieve these new ground rules. Many APC applications (e.g., critical dimension, overlay, etc.) utilize post process metrology data with actual process conditions to provide feedback for new optimized process conditions (e.g., dose, overlay corrections, etc.).
In many cases the feedback systems utilize rules that scan historical metrology data looking for the best available data to use for parameter optimization. Typically, these “rules based” systems will scan the historical data multiple times, modifying the inclusion criteria for each pass, until an acceptable data set is met. For instance, the first rule may look for data resulting from lots processed with a particular reticle, while a second rule may look for data resulting from lots of a certain technology, etc.
The data set obtained from the first pass generally results in the highest confidence feedback calculations. Calculations based on data sets obtained from additional searches typically result in decreasing confidence, since the lower confidence data sets typically are forced to include data from a broader array of products or tools in order to obtain an adequate data set.
Post process metrology data is collected on a sample of the products run through the process step utilizing the APC system. The size of the sample is based on the process capability (Cpk) of the product or selected types of products run through the process step. In current APC applications, a single Cpk value is calculated for a process that in practice runs product from high and low confidence calculations. For instance, a first lot of wafers may be processed according to a first APC process rule, while a second lot of wafers may be processed at a different time according to a second APC process rule. Both lots, however, will be sampled for a metrology step according to a single Cpk value.
In general, the higher the Cpk value, the smaller the sampling requirement. Given the costs involved with collecting metrology data, significant savings can be incurred with increased metrology skipping. Accordingly, a need exists for optimizing the metrology sampling process in APC applications.